Wind turbines commonly used to supply electricity into the electrical grid generally comprise a rotor with a rotor hub and a plurality of blades. The rotor is set into rotation under the influence of the wind on the blades. The rotation of the rotor shaft either directly drives the generator rotor (“directly driven”) or through the use of a gearbox.
A variable speed wind turbine may typically be controlled by varying the generator torque and the pitch angle of the blades. As a result, aerodynamic torque, rotor speed and electrical power will vary.
A common prior art control strategy of a variable speed wind turbine is described with reference to FIG. 1. In FIG. 1, the operation of a typical variable speed wind turbine is illustrated in terms of the pitch angle (β), the electrical power generated (P), the generator torque (M) and the rotational velocity of the rotor (ω), as a function of the wind speed. The curve representing the electrical power generated as a function of wind speed is typically called a power curve.
In a first operational range, from the cut-in wind speed to a first wind speed (e.g. approximately 5 or 6 m/s), the rotor may be controlled to rotate at a substantially constant speed that is just high enough to be able to accurately control it. The cut-in wind speed may be e.g. approximately 3 m/s.
In a second operational range, from the first wind speed (e.g. approximately 5 or 6 m/s) to a second wind speed (e.g. approximately 8.5 m/s), the objective is generally to maximize power output while maintaining the pitch angle of the blades constant so as to capture maximum energy. In order to achieve this objective, the generator torque and rotor speed may be varied so as keep the tip speed ratio λ (tangential velocity of the tip of the rotor blades divided by the prevailing wind speed) constant so as to maximize the power coefficient Cp.
In order to maximize power output and keep Cp constant at its maximum value, the rotor torque may be set in accordance with the following equation:
T=k·ω2, wherein k is a constant, and ω is the rotational speed of the generator. In a direct drive wind turbine, the generator speed substantially equals the rotor speed. In a wind turbine comprising a gearbox, normally, a substantially constant ratio exists between the rotor speed and the generator speed.
In a third operational range, which starts at reaching nominal rotor rotational speed and extends until reaching nominal power, the rotor speed may be kept constant, and the generator torque may be varied to such effect. In terms of wind speeds, this third operational range extends substantially from the second wind speed to the nominal wind speed e.g. from approximately 8.5 m/s to approximately 11 m/s.
In a fourth operational range, which may extend from the nominal wind speed to the cut-out wind speed (for example from approximately 11 m/s to 25 m/s), the blades may be rotated (“pitched”) to maintain the aerodynamic torque delivered by the rotor substantially constant. In practice, the pitch may be actuated such as to maintain the rotor speed substantially constant. At the cut-out wind speed, the wind turbine's operation is interrupted.
In the first, second and third operational ranges, i.e. at wind speeds below the nominal wind speed (the sub-nominal zone of operation), the blades are normally kept in a constant pitch position, namely the “below rated pitch position”. Said default pitch position may generally be close to a 0° pitch angle. The exact pitch angle in “below rated” conditions however depends on the complete design of the wind turbine.
The before described operation may be translated into a so-called power curve, such as the one shown in FIG. 1. Such a power curve may reflect the optimum operation of the wind turbine under steady-state conditions. However, in non-steady state (transient) conditions, the operation may not necessarily be optimum.
In modern variable speed wind turbines, this kind of control or variations on this idea are generally implemented. The control implemented is thus quite different from e.g. active stall and passive stall wind turbines. In active stall machines, above the nominal wind speeds, the blades are pitched so as to cause stall and thus reduce the aerodynamic torque. In passive stall machines, the blades are not rotated but instead are designed and mounted such that stall automatically occurs at higher wind speeds.
As further background, basic aerodynamic behaviour of (the blades of) a wind turbine is explained with reference to FIGS. 2a-2c. 
In FIG. 2a, a profile of a wind turbine blade is depicted in operation. The forces generated by the aerodynamic profile are determined by the wind that the profile “experiences”, the effective wind speed Ve. The effective wind speed is composed of the axial free stream wind speed Va and the tangential speed of the profile Vt The tangential speed of the profile Vt is determined by the instantaneous rotor speed ω and the distance to the centre of rotation of the profile, the local radius r, i.e. Vt=ω·r.
The axial free stream wind speed Va is directly dependent on the wind speed Vw, and on the speed of the wind downstream from the rotor Vdown, that is Va=½(Vw+Vdown). The axial free stream wind speed may e.g. be equal to approximately two thirds of the wind speed Vw.
The resultant wind flow, or effective wind speed Ve, generates lift L and drag D on the blade. A blade may theoretically be divided in an infinite number of blade sections, each blade section having its own local radius and its own local aerodynamic profile. For any given rotor speed, the tangential speed of each blade section will depend on its distance to the rotational axis of the hub (herein referred to as local radius).
The lift generated by a blade (section) depends on the effective wind speed Ve, and on the angle of attack of the blade (section) α, in accordance with the following formula:
      L    =                  1        2            ⁢              ρ        ·                  C          L                    ⁢                        V          e          2                ·        S              ,wherein ρ is the air density, Ve is the effective wind speed, CL is the lift coefficient (dependent on the angle of attack α and on the form of the aerodynamic profile of the blade section), and S is the surface of the blade section.
Similarly, the drag D generated by a blade section can be determined in accordance with the following equation:
      D    =                  1        2            ⁢              ρ        ·                  C          D                    ⁢                        V          e          2                ·        S              ,wherein CD is the drag coefficient dependent on the angle of attack α and on the form of the aerodynamic profile of the blade section.
For an entire wind turbine blade, the contribution to lift and drag of each blade section should be summed to arrive at the total drag and lift generated by the blade. The resultant wind force on a blade is represented by reference sign F in FIG. 2a. The resultant wind force may be seen to be composed of Lift (L) and drag (D), as the wind forces relatively perpendicular to the effective wind direction and in the plane of the effective wind direction. Alternatively, as explained later on, the resultant wind force may also be decomposed in a force Normal (N), i.e. perpendicular to the plane of the chord and a force in the plane of the chord C. And further alternatively, the resultant wind force may also be decomposed in a force in the plane of rotation (In) and a force perpendicular to the plane of rotation (Out).
Both the drag coefficient CD and the lift coefficient CL depend on the profile or the blade section and vary as a function of the angle of attack of the blade section. The angle of attack α may be defined as the angle between the chord line of a profile (or blade section) and the vector of the effective wind flow, see FIG. 2a. 
FIG. 2b illustrates in a very general manner how the static lift coefficient and drag coefficient may vary as a function of the angle of attack of a blade section. Generally, the lift coefficient (reference sign 21) increases to a certain maximum at a so-called critical angle of attack 23. This critical angle of attack is also sometimes referred to as stall angle. The drag coefficient (reference sign 22) may generally be quite low and starts increasing in an important manner close to the critical angle of attack 23. This rapid change in aerodynamic behaviour of a profile or blade section is linked generally to the phenomenon that the aerodynamic flow around the profile (or blade section) is not able to follow the aerodynamic contour and the flow separates from the profile. The separation causes a wake of turbulent flow, which reduces the lift of a profile and increases the drag significantly.
The exact curves of the lift coefficient and drag coefficient may vary significantly in accordance with the aerodynamic profile chosen. However, in general, regardless of the aerodynamic profile chosen, a trend to increasing lift up until a critical angle of attack and also a rapid increase in drag after a critical angle of attack can be found.
In accordance with FIG. 2a, the force in the plane of the chord generated by a blade section is given by In=L·sin(α+∂)−D·cos(α+∂), wherein ∂ is the pitch angle and α is the angle of attack. The pitch angle may be defined as the angle between the rotor plane and the chord line of a profile. Integrating the in-plane distribution over the radius provides the driving torque. Similarly, the out-of-plane force is given by Out=L·cos(α+∂)+D·sin(α+∂). In FIG. 2a, a possible local twist of a blade section is disregarded.
If instead the decomposition in normal to the chord (N) and in the plane of the chord (C) is chosen, the following equations result: N=L·sin α−D·cos α and C=L·cos α+D·sin α. When the pitch angle of the blade equals zero, the In-plane loads correspond to the loads in the plane of the chord (if twist is disregarded) and similarly the out-of-plane loads substantially correspond to the loads perpendicular to the chord.
It furthermore follows from these equations that for relatively small angles of attack, the loads normal to the plane (N) are substantially equal to the lift forces (L). On the other hand, the loads in plane (C) are quite different from the drag forces (D).
In order to increase the torque generated by the rotor, the angle of attack of any blade section is preferably kept below the critical angle of attack such that lift may be higher and drag may be lower. In the before mentioned first operational range, the angles of attack of the blade sections may be relatively low. In the second operational range, the angles of attack of blade sections (or at least of a representative blade section) may be equal to or close to the angle of attack that gives the best ratio of L/D. In the third operational range, the angles of attack may be higher and closer to the critical angle(s) of attack and thus give corresponding high lift coefficients. In the supra-nominal zone of operation, as the blades are pitched, the angles of attack are reduced and are again further away from the critical angle(s) of attack.
It should be borne in mind that the angle of attack of each blade section depends on the tangential speed of the specific rotor blade section, the wind speed, the pitch angle and the local twist angle of the blade section. The local twist angle of a blade section may generally be considered constant, unless some kind of deformable blade is used. The tangential speed of the rotor blade section depends on the rotor speed (angular velocity of the rotor which is obviously the same for the whole blade and thus for each blade section) and on the distance of the blade section to the rotational axis.
For a given pitch angle, it follows that the angle of attack is determined by the tip speed ratio
  λ  =                    ω        ·        R                    V        w              .  From this, it follows that the torque generated by a rotor blade section may become a rather complicated function of the instantaneous tip speed ratio and the pitch angle of the blade.
The lift and drag curves schematically illustrated in FIG. 2b are “static” curves, i.e. they represent the aerodynamic behaviour of a blade section in steady-state conditions. These curves however do not apply to transient conditions. FIG. 2c schematically illustrates the lift coefficient (CO both for static and dynamic conditions (on the left hand side) and the normal coefficient (CO both for static and dynamic conditions, all as a function of the angle of attack α. It is noted that:
            C      L        =          L                        1          2                ⁢                  ρ          ·                      C            L                          ⁢                              V            e            2                    ·          S                      ,and similarly
            C      D        =          D                        1          2                ⁢                  ρ          ·                      C            L                          ⁢                              V            e            2                    ·          S                      ,            C      N        =          N                        1          2                ⁢                  ρ          ·                      C            L                          ⁢                              V            e            2                    ·          S                      ,            and      ⁢                          ⁢              C        c              =                  C                              1            2                    ⁢                      ρ            ·                          C              L                                ⁢                                    V              e              2                        ·            S                              .      Herein Cc is the “chordal coefficient”.
The curves representing the static conditions are shown in interrupted lines. The dynamic behaviour of a blade section may be different in that stall does not occur until a higher angle of attack. In the example shown in FIG. 2c, under the specific dynamic conditions depicted, stall does not occur until an angle of attack of approximately 19°, whereas in dynamic conditions, stall occurs at an angle of attack of around 12°. Also, when stall occurs, the lift decreases very quickly at a very small increment of the angle of attack. After stall, in dynamic conditions, the lift coefficient is significantly lower than it would be under static conditions. That is, if an angle of attack were infinitely slowly increased for the blade section, stall occurs at an angle of attack of around 12° and if the angle of attack is relatively quickly changed, stall occurs at approximately 19°. In reality, a plurality of different dynamic curves exist depending e.g. on the speed of change of (in this case) the angle of attack.
On the right hand side of FIG. 2c, similar static and dynamic curves are shown for Cn. The curves for both CL and Cn are quite similar.
Dynamic conditions in which the behaviour of aerodynamic profiles of wind turbine blades may be similar to the dynamic curves of FIG. 2c may be found e.g. during a wind gust (a relatively rapid increase in wind speed), in cases of wind shear and/or wind veer (as the blade rotates, it encounters a variation of wind speed and/or wind direction respectively leading to a relatively strong variation of the angle of attack). Also, cases wherein the area swept by the blade may be divided into two very distinct layers are known. As a blade passes from one layer into another, an important variation in wind speed and direction can be experienced by the blade. Under these conditions, the aerodynamic behaviour of a section of the blades may correspond more closely to the dynamic curves then to the static curves.
Stall of blade sections is generally not desirable for the operation of modern wind turbines with a variable speed operation as previously discussed. The concepts of control by active or dynamic stall of blades are known. These are however hardly used in modern wind turbines. In modern variable speed wind turbines, the aerodynamic torque is generally limited by pitch control above nominal wind speed.
At or after stall, the aerodynamic lift decreases, whereas the drag increases. Roughly speaking, this means that particularly the loads that are “useless” for the operation of the wind turbine increase, because the loads occurring at or after stall are mainly out-of-plane loads which do not contribute to the aerodynamic torque of the rotor. However, the whole wind turbine structure including blades, rotor, nacelle and tower naturally need to withstand these loads. It is thus generally desired to avoid these useless loads by avoiding stall.
This is even more the case for stall occurring under “dynamic” conditions: the drop in lift is more serious in dynamic conditions, than in static conditions.